(1) Field of the Invention
The present invention relates to bonding structures for optical members, particularly, to bonding structures in which plural optical members such as a laser diode and an optical waveguide substrate are bonded to a mount, while optical axes of the plural optical members are aligned with each other in an order of submicrons (i.e. not more than 0.2 xcexcm). The invention also relates to a method for producing such bonding structures.
(2) Related Art Statement
Second Harmonic Generation (SHG) devices of a quasi phase-matched (QPM) type using optical waveguides in which a periodical domain inversion structure is formed in a single crystal of lithium niobate or lithium tantalate have been used as light sources for blue lasers to be used in optical pickups, for example. Such devices may be applied to a wide region including optical disc memories, medical uses, optochemical uses, various optical measurements, etc.
In order to produce such devices, it is necessary to prepare an optical waveguide substrate provided with a periodical domain inversion structure, fixing the optical waveguide substrate and a laser diode onto a mount, and aligning an optical axis of the optical waveguide of the optical waveguide substrate with that of the laser diode in an order of submicrons.
As such an assembling method, a so-called active alignment method is available, which is not suitable for mass production because a long time is required to make the assembling. On the other hand, according to a passive alignment method and a semi-passive alignment method, a marker is provided on a surface (mounting face) of an integral substrate made of silicon or silicon carbide, and optical members are aligned with reference to the marker. In this case, they are fixed by resin curing, solder fixing, YAG welding or the like. JP-A 6-338650 describes such a method, for example.
When a semiconductor laser diode and an optical waveguide substrate are to be aligned, bonded and fixed on a mounting face of an integral substrate with reference to a maker, it is necessary that the aligning adjustment is effected in a horizontal (width) direction of the substrate, a gap between the semiconductor laser diode and the optical waveguide substrate is adjusted, and the diode and the optical waveguide substrate are adjusted in height. Among the above adjustments, it is difficult to particularly align the height of an optical axis of an active layer of the diode with that of an optical axis of the optical waveguide at a high precision as measured from the mounting face of the substrate. This precision is required to be not more than 0.2 xcexcm, for example. However, when the diode and the optical waveguide substrate are fixed onto the mounting face of the integral substrate with the resin, it is actually difficult to control variations in thickness of the resin to not more than 0.2 xcexcm.
The reason for the above is that it is difficult to accurately coat a necessary amount of the optical adhesive over the entire bonding face of the mount, depending upon influences of the viscosity of the optical adhesive and the wetting property of the mount, which is likely to cause an uneven optical adhesive layer on the bonding face. Particularly in case that the resin is to be applied and cured after the optical members are aligned with each other, a space to be coated with the resin is small in the state that the optical components are aligned, which makes working difficult and uniform coating very difficult.
There are some reasons why the optical axes-aligning precision for the optical members decreases, following curing and shrinkage. For example, in an embodiment of FIG. 2(a), an optical waveguide substrate 10 is arranged on a mounting face 3b of a unitary substrate 3 with an optical adhesive 5A. In this figure, reference numerals 10a, 10b and 10c are an upper face, a bonding face and a side face of the substrate 10, respectively. A part of the optical adhesive 5A flows out from the side face 10c of the substrate 10 to form meniscuses as shown by 12. If each meniscus 12 non-uniformly shrinks when the optical adhesive is cured and shrunk, the substrate 10 is pulled as shown by an arrow direction A, and inclined, which deteriorates the precision in the optical axis-alignment.
In an example of FIG. 2(b), since an optical adhesive layer 5B is thick, a shrunk amount is large in a direction of an arrow B, which causes variations in the optical axis-aligning precision. In an example of FIG. 2(c), a left side portion of the optical bonding adhesive 5C is thicker, while a right portion is thinner. Consequently, the substrate 10 remains inclined after the adhesive is cured and shrunk.
In order to prevent reduction in precision of the optical axis-alignment which follows the above curing and shrinkage, it is necessary to make the thickness of the optical adhesive thinner and also reduce the curing shrinkage. If the viscosity of the optical adhesive is high, the thickness of the optical adhesive is likely to become greater. On the other hand, it is easy to reduce the coated thickness of the optical adhesive having a lower viscosity, but its curing shrinkage rate is often large. Owing to these factors, when optical members are to be mass produced, it is difficult to align the optical axes of the optical members two-dimensionally in a precision order of submicrons with an enhanced yield.
It is an object of the present invention, which is directed to a bonding structure for optical members including a first optical member, a second optical member, and an integral substrate having a mounting face on which the first and second optical components are mounted, with at least the first optical member being bonded to the integral substrate, to prevent reduction in the optical axes-alignment precision of the first and second optical members following curing and shrinkage of the optical adhesive. It is a further object of the present invention to reduce variations in light amount on applications of temperature cycles.
The present invention is directed to the bonding structure for optical members including a first optical member, a second optical member, and an integral substrate having a mounting face on which the first and second optical members are mounted. At least the first optical member is bonded to the integral substrate, and a bonding face of the first optical member is bonded to the mounting face of the integral substrate with a cured and shrunk optical adhesive in such a state that an optical axis of the first optical member is aligned with that of the second optical member at a precision of not more than 1xcexcm. A groove is formed in the bonding region of the mounting face of the integral substrate bonded with the first optical member, and at least part of the optical adhesive is filled into the groove.